Baby Milestones: Don't Compare to Other Kids!
Of course, it's reasonable that you would want to ensure that your kid is on track and doesn't have any developmental delays. But there is a wide range of what is considered normal child development. And kids develop at their own pace. Babies tend to focus on one area at a time. So the baby who masters crawling early might talk on the later side. And the baby who is talking by age one may barely crawl. Try not to freak out.
Rest assured that there is no indication that kids who hit their developmental milestones early will be more successful later in life. In other words, just because your kid started walking at 15 months doesn't mean he won't grow up to be an athlete and your late talker could turn out to be a Rhodes scholar.
If you are truly concerned that your child isn't reaching his milestones when he should, then speak with your pediatrician.